Method and apparatus for image encoding/decoding

ABSTRACT

A method and an apparatus for image encoding/decoding are disclosed. The apparatus includes: a block decomposer for decomposing a current block into a plurality of sub-blocks; and an intra prediction encoder for performing an intra prediction encoding by referring to already encoded and decoded adjacent pixel information of each sub-block, based on an intra prediction mode equal to an intra prediction mode of the current block, thereby generating a bit stream for the current block. In image encoding and image decoding, when a current block to be encoded or decoded is predicted, the disclosed method and apparatus can improve the accuracy of the prediction, which can provide an image with a satisfactory reproduction quality by improved coding efficiency.

TECHNICAL FIELD

The present disclosure relates to a method and apparatus for imageencoding/decoding. More particularly, the present disclosure relates toan image encoding/decoding method and apparatus, which can improve theaccuracy of a prediction on a current block to be encoded or decoded inimage encoding/decoding, thereby providing a satisfactory imagereproduction quality by improved coding efficiency.

BACKGROUND ART

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Moving Picture Experts Group (MPEG) and Video Coding Experts Group(VCEG) have developed an improved and excellent video compressiontechnology over existing MPEG-4 Part 2 and H.263 standards. The newstandard is named H.264/AVC (Advanced Video Coding) and was releasedsimultaneously as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264.Such H.264/AVC (hereinafter referred to as ‘H.264’) uses a spatialpredictive coding method, which is different from conventional videocoding-related international standards such as MPEG-1, MPEG-2, MPEG-4Part2 Visual and the like.

Conventional video coding methods use “intra prediction” forcoefficients transformed in discrete cosine transform domain (or DCTtransform domain) to seek higher encoding efficiency, resulting indegradation of the subjective video quality at low band transmission bitrates. However, H.264 adopts a method of encoding based on a spatialintra prediction in a spatial domain rather than in a transform domain.

An encoder using an encoding method based on the conventional spatialintra prediction predicts pixels of a block to currently encode frompixels in the previous blocks that have already undergone encoding andhaving been reproduced, encodes just the differences of the predictedblock pixel values from pixel values of the actual block, and transmitsthe encoded difference information to a decoder. At this time, theencoder may either transmit parameters needed for prediction to thedecoder or previously share the parameters needed for prediction withthe decoder, so as to enable the decoder to predict the current block.Meanwhile, the decoder predicts information of a desired current blockto be currently decoded by using information of neighbor blocks alreadydecoded and reproduced, and reconstructs and reproduces the desiredcurrent block based on a sum of the predicted information and thedifference information transmitted from the encoder. At this time also,if the decoder has received parameters need for the prediction from theencoder, the decoder uses the received parameters in predicting anddecoding the current block.

However, in a prediction using the intra prediction according to theconventional video coding and video decoding, pixels of a current blockto be currently encoded are predicted by referring to the alreadyreconstructed information of adjacent pixels within a neighbor blockaround the current block (usually a block located at the left or abovethe current block). Here, the pixels of the current block to be encodedand the adjacent pixels of the neighbor block referred to in order topredict the current block may be spaced apart from each other by aconsiderably long distance. At this time, the accuracy of the predictionmay be degraded too much, which may lower the quality of video encoding.As a result, the low quality video encoding may yield an unsatisfactoryreproduction state when the image encoded with the low quality isrestored and reproduced.

DISCLOSURE Technical Problem

Therefore, the present disclosure has been made in an effort to improvethe accuracy of prediction on a current block to be encoded or decodedin encoding or decoding an image, and thus provide a satisfactory imagereproduction quality for the image by improved coding efficiency.

Technical Solution

One aspect of the present disclosure provides an apparatus for encodingan image, including: a block decomposer for decomposing a current blockinto a plurality of sub-blocks; and an intra prediction encoder forperforming an intra prediction encoding by referring to already encodedand decoded adjacent pixel information of each sub-block, based on anintra prediction mode equal to an intra prediction mode of the currentblock, thereby generating a bit stream for the current block.

Another aspect of the present disclosure provides a method of encodingan image, including: decomposing a current block into a plurality ofsub-blocks; and generating a bit stream for the current block byperforming an intra prediction encoding by referring to already encodedand decoded adjacent pixel information of each sub-block, based on anintra prediction mode equal to an intra prediction mode of the currentblock.

Yet another aspect of the present disclosure provides an apparatus fordecoding an image, including: a decoder for decoding a received bitstream, so as to extract residual sub-blocks and an intra predictionmode for a plurality of sub-blocks decomposed from a current block; adequantizer for dequantizing the extracted residual sub-block; aninverse transform unit for performing an inverse transform on thedequantized residual sub-block; an intra predictor for performing anintra prediction according to the intra prediction mode with referenceto adjacent pixel information for each sub-block already decoded andreconstructed, thereby generating predicted sub-blocks for the pluralityof sub-blocks; an adder for adding the inverse-transformed residualsub-blocks and the predicted sub-blocks to reconstruct the plurality ofsub-blocks; and a block combiner for reconstructing the current block byscanning the plurality of reconstructed sub-blocks according to asequence of a zigzag sequence, wherein the intra prediction mode for theplurality of sub-blocks is equal to an intra prediction mode of thecurrent block.

Yet another aspect of the present disclosure provides a method ofdecoding an image, including: decoding a received bit stream, so as toextract residual sub-blocks and an intra prediction mode for a pluralityof sub-blocks decomposed from a current block; dequantizing theextracted residual sub-block;

performing an inverse transform on the dequantized residual sub-block;generating predicted sub-blocks for the plurality of sub-blocks byperforming an intra prediction according to the intra prediction modewith reference to adjacent pixel information for each sub-block alreadydecoded and reconstructed; reconstructing the plurality of sub-blocks byadding the inverse-transformed residual sub-blocks and the predictedsub-blocks; and restoring the current block by scanning the plurality ofreconstructed sub-blocks according to a sequence of a zigzag sequence,wherein the intra prediction mode for the plurality of sub-blocks isequal to an intra prediction mode of the current block.

Advantageous Effects

In image encoding and image decoding according to an embodiment of thepresent disclosure, when a current block to be encoded or decoded ispredicted, the current block is decomposed into sub-blocks and each ofthe sub-blocks is then intra-predicted by referring to adjacent pixelinformation of pixels adjacent to the corresponding sub-block, insteadof performing an intra prediction on each decomposed sub-block byreferring to adjacent pixel information of surrounding blocks adjacentto the current block. As a result, the image encoding method and imagedecoding method according to the present disclosure can improve theaccuracy of the prediction, which can provide an image with asatisfactory reproduction quality by improved coding efficiency.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram of an image encoding apparatusaccording to an aspect of the present disclosure;

FIG. 2 is a flowchart of an image encoding according to an embodiment ofthe present disclosure;

FIG. 3 illustrates an example of a current block and a plurality ofsub-blocks decomposed from the current block for application of an imageencoding method according an embodiment of the present disclosure;

FIG. 4 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a horizontal mode;

FIG. 5 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a vertical mode;

FIG. 6 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a DC mode;

FIG. 7 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a horizontal mode;

FIG. 8 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a vertical mode;

FIG. 9 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a DC mode;

FIGS. 10A and 10B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a horizontal mode;

FIGS. 11A and 11B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a vertical mode;

FIGS. 12A and 10B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a DC mode;

FIG. 13 is a view for describing a process of generating a residualblock for a certain sub-block on which an intra prediction has beenperformed;

FIG. 14 is a schematic block diagram of an image decoding apparatus 1400according to an embodiment of the present disclosure; and

FIG. 15 is a flowchart of an image decoding method according to anembodiment of the present disclosure.

MODE FOR INVENTION

Hereinafter, aspects of the present disclosure will be described indetail with reference to the accompanying drawings. In the followingdescription, the same elements will be designated by the same referencenumerals although they are shown in different drawings. Further, in thefollowing description of the present disclosure, a detailed descriptionof known functions and configurations incorporated herein will beomitted when it may make the subject matter of the present disclosurerather unclear.

Also, in describing the components of the present disclosure, there maybe terms used like first, second, A, B, (a), and (b). These are solelyfor the purpose of differentiating one component from the other but notto imply or suggest the substances, order or sequence of the components.If a component were described as ‘connected’, ‘coupled’, or ‘linked’ toanother component, they may mean the components are not only directly‘connected’, ‘coupled’, or ‘linked’ but also are indirectly ‘connected’,‘coupled’, or ‘linked’ via a third component.

FIG. 1 is a schematic block diagram of an image encoding apparatus 100according to an aspect of the present disclosure.

Referring to FIG. 1, the image encoding apparatus 100 according to anaspect of the present disclosure includes a block decomposer 110 fordecomposing a current block to be encoded in an input original imageinto a plurality of sub-blocks, and an intra prediction encoder 120 forgenerating a bit stream for the current block by performing an intraprediction encoding by referring to already encoded or decoded adjacentpixel information of each sub-block.

The block divider 110 may divide the current block into multiplesub-blocks based on the unit of frequency conversion.

Referring to FIG. 1, the intra prediction encoder 120 includes: an intrapredictor 121 for selecting one sub-block among a plurality ofsub-blocks according to a sequence of a raster scan or a zigzagsequence, and generating a predicted sub-block by performing an intraprediction for the selected sub-block with reference to adjacent pixelinformation of each corresponding sub-block based on an intra predictionmode equal to the intra prediction mode of the current block; asubtractor 122 for generating a residual sub-block by calculating adifference between the selected single sub-block and a correspondingpredicted sub-block; a transform unit 123 for performing a DiscreteCosine Transform (DCT) on the generated residual sub-block; a quantizer124 for quantizing the transformed residual sub-block; and an encoder125 for encoding the quantized residual sub-block. Here, each of thedivided sub-blocks is a transform-based block in the transform unit 123,and may be a frequency transform-based block.

The sequence in which a plurality of sub-blocks areintra-prediction-encoded may be a sequence of a raster scan or a zigzagsequence. It may be more efficient to perform theintra-prediction-encoding by selecting one sub-block from a plurality ofsub-blocks according to the sequence of a raster scan than the zigzagsequence. Although only two cases including the sequence of a rasterscan and the zigzag sequence are discussed as the sequences by which aplurality of sub-blocks can be intra-prediction-encoded in the presentdisclosure, it is also possible to change the sequence according to thecharacteristic of the image.

The transform unit 123 described above may perform a Discrete

Cosine Transform (DCT). The encoder 125 described above may perform anentropy encoding, in which the length of a code indicating a symbolchanges according to the probability that the symbol occurs.

The intra prediction encoder 120 described above may further include adequantizer 126 for dequantizing a quantized residual sub-block; aninverse transform unit 127 for performing an inverse transform on thedequantized residual sub-block; an intra-compensator 128 for performingan intra-compensation on the inverse-transformed residual sub-block,thereby generating a reference block; and a reference block storage unit129 for storing the generated reference block.

The reference blocks stored in the reference block storage unit 129described above contain adjacent pixel information of each sub-block,which will be referred to when an intra prediction is performed for asub-block to be selected after the single sub-block selected frommultiple sub-blocks according to the sequence of a raster scan or thezigzag sequence.

The intra prediction mode of the current block mentioned above may be,for example, one of a vertical mode, a horizontal mode, and a DirectCurrent (DC) mode in the 16×16 intra prediction mode.

The intra prediction of each sub-block employs the same intra predictionmode as the intra prediction mode of the current block. For example,based on an assumption that the current block is a 16×16 block and eachof the sub-blocks decomposed from the current block is a 4×4 block, whenthe 16×16 intra prediction mode for the current block is a verticalmode, the 4×4 intra prediction mode for each sub-block is also avertical mode. Further, when the 16×16 intra prediction mode for thecurrent block is a horizontal mode, the 4×4 intra prediction mode foreach sub-block is also a horizontal mode. When the 16×16 intraprediction mode for the current block is a DC mode, the 4×4 intraprediction mode for each sub-block is also a DC mode.

FIG. 2 is a flowchart of an image encoding according to an embodiment ofthe present disclosure.

Referring to FIG. 2, an image encoding method performed by an imageencoding apparatus 100 according to an embodiment of the presentdisclosure includes: a block decomposing step (S200) of decomposing acurrent block to be encoded in an input original image into a pluralityof sub-blocks; and an intra prediction encoding step (S202) ofgenerating a bit stream for the current block by performing an intraprediction encoding by referring to reference pixel information for eachsub-block already encoded and decoded.

The intra prediction encoding step (S202) may include: a step (S2020) ofselecting a single sub-block among a plurality of sub-blocks accordingto a sequence of a raster scan or a zigzag sequence, and generating apredicted sub-block by performing an intra prediction for the selectedsingle sub-block with reference to adjacent pixel information of eachcorresponding sub-block based on an intra prediction mode equal to theintra prediction mode of the current block; a step (S2022) of generatinga residual sub-block by calculating a difference between the selectedsingle sub-block and a corresponding predicted sub-block; a step (S2024)of transforming the generated residual sub-block; a step (S2026) ofquantizing the transformed residual sub-block; and a step (S2028) ofencoding the quantized residual sub-block.

After the quantization step (S2026), the intra prediction encoding step(S202) may further include the steps of: dequantizing a quantizedresidual sub-block; performing an inverse transform on the dequantizedresidual sub-block; performing an intra-compensation on theinverse-transformed residual sub-block, thereby generating a referenceblock; and storing the generated reference block. Those further includedsteps are performed in parallel with the intra prediction step (S2020),the residual sub-block generating step (S2022), the transform step(S2024), the quantization step (S2026), and the encoding step (S2028).That is to say, the steps added in order to generate and store thereference block may be simultaneously performed with the intraprediction encoding process for the sub-block.

In order to apply the image encoding method according an embodiment ofthe present disclosure as described above, an original image isdecomposed into Macro Blocks (MBs) each having a predetermined size. Inthe present disclosure, the decomposed macro block is called a “currentblock”.

Further, in the image encoding method according an embodiment of thepresent disclosure, a current block, which is a 16×16 macro block, isdecomposed into a plurality of Sub-Blocks (SBs), each of which is abasic unit in the intra prediction encoding. Further, each sub-blockincludes a plurality of pixels, and each pixel contains information of acorresponding part in the original image.

The current block, sub-block, and pixel as described above will now bedescribed again with reference to FIG. 3 showing an example of a 16×16block.

FIG. 3 illustrates an example of a current block and a plurality ofsub-blocks decomposed from the current block for the application of animage encoding method according an embodiment of the present disclosure.

In FIG. 3, a current block applied to an image encoding according anembodiment of the present disclosure is a 16×16 macro block, which isidentified by a box having a contour defined by a thick solid line.

Further, in FIG. 3, the current block, which is a 16×16 macro blockdefined by a thick solid line, is decomposed into 16 sub-blocks eachhaving a 4×4 block size. The 16 decomposed sub-blocks are named “SB 1,SB 2, SB 3, . . . , and SB 16” and are indicated by boxes defined bybroken lines.

Referring to FIG. 3, each sub-block includes 16 pixels, and each pixelcontains information on the image.

In FIGS. 3, V(0) to V(15) and H(0) to H(15) correspond to pixelinformation on adjacent macro blocks of the current block (macro block),and the pixel information on the adjacent macro blocks correspond topixel information of already encoded and decoded macro blocks.

By using the current block including the 16 sub-blocks (SB 1, SB 2, SB3, . . . , and SB 16) illustrated as an example in FIG. 3, an imageencoding according to an embodiment of the present disclosure will nowbe described with reference to FIGS. 1 and 2.

In an image encoding according to an embodiment of the presentdisclosure, a current block having a 16×16 block size is decomposed into16 sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16), and an intraprediction encoding is then performed for each of the sub-blocksaccording to a predetermined sequence. Here, the predetermined sequencemay be a sequence of a raster scan or a zigzag sequence.

In other words, the 16 sub-blocks are scanned according to a sequence ofa raster scan or a zigzag sequence, and an intra prediction encoding isthen performed for a first sub-block among the scanned sub-blocks. Then,a next sub-block is determined among the 16 sub-blocks scanned accordingto the sequence of a raster scan or the zigzag sequence, and an intraprediction encoding is then performed for the next sub-block.Thereafter, the other sub-blocks are also subjected to the intraprediction encoding according to the sequence of a raster scan or thezigzag sequence. After the intra prediction encoding is completelyperformed for all the decomposed sub-blocks, a bit stream for thecurrent block is generated by using the result of the intra predictionencoding for each sub-block.

The intra prediction encoding described above corresponds to a processperformed for each sub-block, and may include a series of stepsincluding an intra prediction, a transform, a quantization, and anencoding.

In the intra prediction step included in the intra prediction encodingperformed for each sub-block, an intra prediction mode is selected forthe intra prediction. In the present disclosure, the same intraprediction mode as the intra prediction mode of the current block isselected as an intra prediction mode for each sub-block.

For example, when the intra prediction mode of the current block is ahorizontal mode, the intra prediction mode of the decomposed sub-blocksis also a horizontal mode. When the intra prediction mode of the currentblock is a vertical mode, the intra prediction mode of the decomposedsub-blocks is also a vertical mode. Also, when the intra prediction modeof the current block is a DC mode, the intra prediction mode of thedecomposed sub-blocks is also a DC mode.

An image encoding through an intra prediction encoding of each sub-blockwhen the intra prediction mode of the current block is a horizontal modewill be described in detail with reference to FIGS. 4 and 7, an imageencoding through an intra prediction encoding of each sub-block when theintra prediction mode of the current block is a vertical mode will bedescribed in detail with reference to FIGS. 5 and 8, and an imageencoding through an intra prediction encoding of each sub-block when theintra prediction mode of the current block is a DC mode will bedescribed in detail with reference to FIGS. 6 and 9. In the meantime,the intra prediction encoding shown in FIGS. 4, 5, and 6 corresponds toan intra prediction encoding according to a sequence of a raster scan,while the intra prediction encoding shown in FIGS. 7, 8, and 9corresponds to an intra prediction encoding according to a zigzagsequence.

FIG. 4 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a horizontal mode.

Referring to FIG. 4, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the sequence of araster scan. That is, the intra prediction encoding is performed in thesequence of sub-blocks SB 1, SB 2, SB 3, SB 4, SB 5, SB 6, SB 7, SB 8,SB 9, SB 10, SB 11, SB 12, SB 13, SB 1 SB 15, and SB 16.

In FIG. 4, since it is assumed that the intra prediction mode of thecurrent block is a horizontal mode, the intra prediction mode of thesub-block is also the horizontal mode. Therefore, the image encodingapparatus 100 performs the intra prediction in the horizontal mode oneach sub-block by referring to the adjacent pixel information.

As shown in FIG. 4, by referring to adjacent pixel information (H0),(H1), (H2), and (H3), which are adjacent to the first sub-block (SB 1),among pixel information of macro blocks adjacent to the current block,the image encoding apparatus 100 performs an intra prediction encodingon the first sub-block (SB 1), and simultaneously generates and stores areference block. The stored reference block includes adjacent pixelinformation, which is referred to during an intra prediction encodingprocess for another sub-block.

In FIG. 4, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16”. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the sequence of a raster scan.

Here, the adjacent pixel information referred to during the intraprediction encoding on SB 2 includes pixel information “1-4, 1-8, 1-12,and 1-16” from among the pixel information included in the referenceblock stored during the intra prediction encoding on SB 1. The reasonwhy “1-4, 1-8, 1-12, and 1-16” are used for reference as adjacent pixelinformation from among the pixel information included in the referenceblock is that the intra prediction encoding on SB 2 is performed in thehorizontal mode.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 3 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 3 includes pixel information “2-4, 2-8, 2-12,and 2-16”, which are adjacent to SB 3.

Following SB 3, the image encoding apparatus 100 performs an intraprediction encoding on SB 4 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 4 includes pixel information “3-4, 3-8, 3-12,and 3-16”, which are adjacent to SB 4.

Following SB 4, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 5 includes pixel information “(H4), (H5),(H6), and (H7)”, which are adjacent to SB 5.

As in the intra prediction encoding process for SB 1, SB 2, SB 3, SB 4,and SB 5 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thesequence of a raster scan by referring to already encoded and decodedadjacent pixel information (pixel information having a second number of4, 8, 12, or 16 in indices of pixel information of each sub-block) in anadjacent sub-block or already encoded and decoded adjacent pixelinformation in an adjacent macro block.

FIG. 5 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a vertical mode.

Referring to FIG. 5, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the sequence of araster scan. That is, the intra prediction encoding is performed in thesequence of sub-blocks SB 1, SB 2, SB 3, SB 4, SB 5, SB 6, SB 7, SB 8,SB 9, SB 10, SB 11, SB 12, SB 13, SB 14, SB 15, and SB 16.

In FIG. 5, since it is assumed that the intra prediction mode of thecurrent block is a vertical mode, the intra prediction mode of thesub-block is also the vertical mode. Therefore, the image encodingapparatus 100 performs the intra prediction in the vertical mode on eachsub-block by referring to the adjacent pixel information.

As shown in FIG. 5, by referring to adjacent pixel information (V0),(V1), (V2), and (V3), which are adjacent to the first sub-block (SB 1),among pixel information of macro blocks adjacent to the current block,the image encoding apparatus 100 performs an intra prediction encodingon the first sub-block (SB 1), and simultaneously generates and stores areference block. The stored reference block includes adjacent pixelinformation, which is referred to during an intra prediction encodingprocess for another sub-block.

In FIG. 5, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16˜. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 2 includes pixel information “(V4), (V5),(V6), and (V7)”, which are adjacent to the SB 2, from among the pixelinformation of the macro blocks adjacent to the current block.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 3 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 3 includes pixel information “(V8), (V9),(V10), and (V11)”, which are adjacent to the SB 3, from among the pixelinformation of the macro blocks adjacent to the current block.

Following SB 3, the image encoding apparatus 100 performs an intraprediction encoding on SB 4 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 4 includes pixel information “(V12), (V13),(V14), and (V15)”, which are adjacent to the SB 4, from among the pixelinformation of the macro blocks adjacent to the current block.

Following SB 4, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the sequence of a raster scan.

Here, the adjacent pixel information referred to during the intraprediction encoding on SB 5 includes pixel information “1-13, 1-14,1-15, and 1-16” from among the pixel information included in thereference block stored during the intra prediction encoding on SB 1,which is a sub-block adjacent to SB 5.

As in the intra prediction encoding process for SB 1, SB 2, SB 3, SB 4,and SB 5 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thesequence of a raster scan by referring to already encoded and decodedadjacent pixel information (pixel information having a second number of13, 14, 15, or 16 in indices of pixel information of each sub-block) inan adjacent sub-block or already encoded and decoded adjacent pixelinformation in an adjacent macro block.

FIG. 6 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a sequence of a raster scan whenthe intra prediction mode of the current block is a DC mode.

Referring to FIG. 6, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the sequence of araster scan. That is, the intra prediction encoding is performed in thesequence of sub-blocks SB 1, SB 2, SB 3, SB 4, SB 5, SB 6, SB 7, SB 8,SB 9, SB 10, SB 11, SB 12, SB 13, SB 14, SB 15, and SB 16.

In FIG. 6, since it is assumed that the intra prediction mode of thecurrent block is a DC mode, the intra prediction mode of the sub-blockis also the DC mode. Therefore, the image encoding apparatus 100performs the intra prediction in the DC mode on each sub-block byreferring to the adjacent pixel information.

As shown in FIG. 6, by referring to adjacent pixel information (HO),(H1), (H2), (H3), (V0), (V1), (V2), (V3) and M, which are adjacent tothe first sub-block (SB 1), among pixel information of macro blocksadjacent to the current block, the image encoding apparatus 100 performsan intra prediction encoding on the first sub-block (SB 1), andsimultaneously generates and stores a reference block. The storedreference block includes adjacent pixel information, which is referredto during an intra prediction encoding process for another sub-block.

In FIG. 6, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16˜. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 2 includes pixel information “1-4, 1-8, 1-12,1-16, (V4), (V5), (V6), (V7), and (V3)”. From among the pixelinformation “1-4, 1-8, 1-12, 1-16, (V4), (V5), (V6), (V7), and (V3)”,“1-4, 1-8, 1-12, and 1-16” correspond to pixel information in SB 1,which is a sub-block adjacent to SB 2, and “(V4), (V5), (V6), (V7), and(V3)” correspond to pixel information adjacent to SB 2 from among thepixel information on the macro blocks adjacent to the current block. Thereason why “1-4, 1-8, 1-12, 1-16, (V4), (V5), (V6), (V7), and (V3)” areused for reference as adjacent pixel information in the intra predictionencoding on SB 2 as described above is that the intra predictionencoding on SB 2 is performed in the DC mode.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 3 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 3 includes pixel information “2-4, 2-8, 2-12,2-16, (V8), (V9), (V10), (V11), and (V7)”.

Following SB 3, the image encoding apparatus 100 performs an intraprediction encoding on SB 4 according to the sequence of a raster scan.Here, the adjacent pixel information referred to during the intraprediction encoding on SB 4 includes pixel information “3-4, 3-8, 3-12,3-16, (V12), (V13), (V14), (V15), and (V11)”.

Following SB 4, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the sequence of a raster scan.

Here, the adjacent pixel information referred to during the intraprediction encoding on SB 5 includes pixel information “(H4), (H5),(H6), (H7), 1-13, 1-14, 1-15, 1-16, and (H3)”.

As in the intra prediction encoding process for SB 1, SB 2, SB 3, SB 4,and SB 5 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thesequence of a raster scan by referring to already encoded and decodedadjacent pixel information (pixel information on pixels located at theright end and the lower end of each sub-block) in an adjacent sub-blockor already encoded and decoded adjacent pixel information in an adjacentmacro block.

FIG. 7 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a horizontal mode.

Referring to FIG. 7, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the zigzag sequence.That is, the intra prediction encoding is performed in the sequence ofsub-blocks SB 1, SB 2, SB 5, SB 9, SB 6, SB 3, SB 4, SB 7, SB 10, SB 13,SB 14, SB 11, SB 8, SB 12, SB 15, and SB 16.

In FIG. 7, since it is assumed that the intra prediction mode of thecurrent block is a horizontal mode, the intra prediction mode of thesub-block is also the horizontal mode. Therefore, the image encodingapparatus 100 performs the intra prediction in the horizontal mode oneach sub-block by referring to the adjacent pixel information.

As shown in FIG. 7, by referring to adjacent pixel information (H0),(H1), (H2), and (H3), which are adjacent to the first sub-block (SB 1),among pixel information of macro blocks adjacent to the current block,the image encoding apparatus 100 performs an intra prediction encodingon the first sub-block (SB 1), and simultaneously generates and stores areference block. The stored reference block includes adjacent pixelinformation, which is referred to during an intra prediction encodingprocess for another sub-block.

In FIG. 7, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16˜. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 2 includes pixel information “1-4, 1-8, 1-12, and 1-16”from among the pixel information included in the reference block storedduring the intra prediction encoding on SB 1. The reason why “1-4, 1-8,1-12, and 1-16” are used for reference as adjacent pixel informationfrom among the pixel information included in the reference block is thatthe intra prediction encoding on SB 2 is performed in the horizontalmode.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 5 includes pixel information “H(4), (H5), (H6), and(H7)”, which are adjacent to SB 5, from among the pixel information onthe macro blocks adjacent to the current block.

Following SB 5, the image encoding apparatus 100 performs an intraprediction encoding on SB 9 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 9 includes pixel information “H(8), (H9), (H10), and(H11)”, which are adjacent to SB 9, from among the pixel information onthe macro blocks adjacent to the current block.

Following SB 9, the image encoding apparatus 100 performs an intraprediction encoding on SB 6 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 6 includes pixel information “5-4, 5-8, 5-12, and 5-16”from among the pixel information included in the reference block storedduring the intra prediction encoding on SB 5.

As in the intra prediction encoding process for SB 1, SB 2, SB 5, SB 9,and SB 6 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thezigzag sequence by referring to already encoded and decoded adjacentpixel information (pixel information having a second number of 4, 8, 12,or 16 in indices of pixel information of each sub-block) in an adjacentsub-block or already encoded and decoded adjacent pixel information inan adjacent macro block.

FIG. 8 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a vertical mode.

Referring to FIG. 8, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the zigzag sequence.That is, the intra prediction encoding is performed in the sequence ofsub-blocks SB 1, SB 2, SB 5, SB 9, SB 6, SB 3, SB 4, SB 7, SB 10, SB 13,SB 14, SB 11, SB 8, SB 12, 15, and SB 16.

In FIG. 8, since it is assumed that the intra prediction mode of thecurrent block is a vertical mode, the intra prediction mode of thesub-block is also the vertical mode. Therefore, the image encodingapparatus 100 performs the intra prediction in the vertical mode on eachsub-block by referring to the adjacent pixel information.

As shown in FIG. 8, by referring to adjacent pixel information (V0),(V1), (V2), and (V3), which are adjacent to the first sub-block SB 1,among pixel information of macro blocks adjacent to the current block,the image encoding apparatus 100 performs an intra prediction encodingon the first sub-block SB 1, and simultaneously generates and stores areference block. The stored reference block includes adjacent pixelinformation, which is referred to during an intra prediction encodingprocess for another sub-block.

In FIG. 8, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16”. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 2 includes pixel information “(V4), (V5), (V6), and(V7)”, which are adjacent to the SB 2, from among the pixel informationof the macro blocks adjacent to the current block.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 5 includes pixel information “1-13, 1-14, 1-15, and 1-16”from among the pixel information included in the reference block storedduring the intra prediction encoding on SB 1, which is a sub-blockadjacent to SB 5. The reason why “1-13, 1-14, 1-15, and 1-16” from amongthe pixel information included in the reference block stored for SB 1 isused for reference as adjacent pixel information is that the intraprediction encoding on SB 5 is performed in the vertical mode.

Following SB 5, the image encoding apparatus 100 performs an intraprediction encoding on SB 9 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 9 includes pixel information of pixel information “5-13,5-14, 5-15, and 5-16” from among the pixel information included in thereference block stored during the intra prediction encoding on SB 5,which is a sub-block adjacent to SB 9.

Following SB 9, the image encoding apparatus 100 performs an intraprediction encoding on SB 6 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 6 includes pixel information “2-13, 2-14, 2-15, and 2-16”from among the pixel information included in the reference block storedduring the intra prediction encoding on SB 2, which is a sub-blockadjacent to SB 6.

As in the intra prediction encoding process for SB 1, SB 2, SB 5, SB 9,and SB 6 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thezigzag sequence by referring to already encoded and decoded adjacentpixel information (pixel information having a second number of 13, 14,15, or 16 in indices of pixel information of each sub-block) in anadjacent sub-block or already encoded and decoded adjacent pixelinformation in an adjacent macro block.

FIG. 9 illustrates an example of an intra prediction encoding of aplurality of sub-blocks according to a zigzag sequence when the intraprediction mode of the current block is a DC mode.

Referring to FIG. 9, in order to encode the current block, the 16decomposed sub-blocks (SB 1, SB 2, SB 3, . . . , and SB 16) aresequentially intra-prediction-encoded according to the zigzag sequence.That is, the intra prediction encoding is performed in the sequence ofsub-blocks SB 1, SB 2, SB 5, SB 9, SB 6, SB 3, SB 4, SB 7, SB 10, SB 13,SB 14, SB 11, SB 8, SB 12, SB 15, and SB 16.

In FIG. 9, since it is assumed that the intra prediction mode of thecurrent block is a DC mode, the intra prediction mode of the sub-blockis also the DC mode. Therefore, the image encoding apparatus 100performs the intra prediction in the DC mode on each sub-block byreferring to the adjacent pixel information.

As shown in FIG. 9, by referring to adjacent pixel information (H0),(H1), (H2), (H3), (V0), (V1), (V2), (V3) and M, which are adjacent tothe first sub-block (SB 1), among pixel information of macro blocksadjacent to the current block, the image encoding apparatus 100 performsan intra prediction encoding on the first sub-block (SB 1), andsimultaneously generates and stores a reference block. The storedreference block includes adjacent pixel information, which is referredto during an intra prediction encoding process for another sub-block.

In FIG. 9, during the intra prediction encoding for the first sub-block(SB 1), pixel information included in the reference block, which hasbeen already encoded, decoded, and stored, is indicated by indices “1-1,1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12, 1-13, 1-14,1-15, and 1-16˜. In each index of “first number-second number”indicating the pixel information, the first number identifies acorresponding sub-block, and the second number identifies acorresponding pixel within the corresponding sub-block.

Following SB 1, the image encoding apparatus 100 performs an intraprediction encoding on SB 2 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 2 includes pixel information “1-4, 1-8, 1-12, 1-16, (V4),(V5), (V6), (V7), and (V3)”, From among the pixel information “1-4, 1-8,1-12, 1-16, (V4), (V5), (V6), (V7), and (V3)”, “1-4, 1-8, 1-12, and1-16” correspond to pixel information in SB 1, which is a sub-blockadjacent to SB 2, and “(V4), (V5), (V6), (V7), and (V3)” correspond topixel information adjacent to SB 2 from among the pixel information onthe macro blocks adjacent to the current block. The reason why “1-4,1-8, 1-12, 1-16, (V4), (V5), (V6), (V7), and (V3)” are used forreference as adjacent pixel information in the intra prediction encodingon SB 2 as described above is that the intra prediction encoding on SB 2is performed in the DC mode.

Following SB 2, the image encoding apparatus 100 performs an intraprediction encoding on SB 5 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 5 includes pixel information “(H4), (H5), (H6), (H7),1-13, 1-14, 1-15, 1-16, and (H3)”. From among the pixel information“(H4), (H5), (H6), (H7), 1-13, 1-14, 1-15, 1-16, and (H3)”, “(H4), (H5),(H6), (H7), and (H3)” correspond to pixel information adjacent to SB 5from among the pixel information on the macro blocks adjacent to thecurrent block, and “1-13, 1-14, 1-15, and 1-16” correspond to pixelinformation in SB 1, which is a sub-block adjacent to SB 5.

Following SB 5, the image encoding apparatus 100 performs an intraprediction encoding on SB 9 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 9 includes pixel information “(H8), (H9), (H10), (H11),5-13, 5-14, 5-15, 5-16, and (H7)”.

Following SB 9, the image encoding apparatus 100 performs an intraprediction encoding on SB 6 according to the zigzag sequence. Here, theadjacent pixel information referred to during the intra predictionencoding on SB 6 includes pixel information “5-4, 5-8, 5-12, 5-16, 2-13,2-14, 2-15, 2-16, and 1-16”. From among the pixel information, “5-4,5-8, 5-12, and 5-16” correspond to pixel information in SB 5, which is asub-block adjacent to SB 6, “2-13, 2-14, 2-15, and 2-16” correspond topixel information in SB 2, which is a sub-block adjacent to SB 6, and“1-16” corresponds to pixel information in SB 1, which is a sub-blockadjacent to SB 6.

As in the intra prediction encoding process for SB 1, SB 2, SB 5, SB 9,and SB 6 described above, the image encoding apparatus 100 performs anintra prediction encoding on the remaining sub-blocks according to thezigzag sequence by referring to already encoded and decoded adjacentpixel information (pixel information on pixels located at the right endand the lower end of each sub-block) in an adjacent sub-block or alreadyencoded and decoded adjacent pixel information in an adjacent macroblock.

The above description with reference to FIGS. 4 to 9 presentsembodiments of the present disclosure, in which the image encodingapparatus 100 intra-prediction-encodes sub-blocks of a current block inthree types of intra prediction modes (horizontal mode, vertical mode,or DC mode) according to a predetermined sequence (sequence of a rasterscan or a zigzag sequence).

Hereinafter, the process of intra prediction encoding for each sub-blockis described in more detail with reference to FIGS. 10A to 13. FIGS. 10Ato 12 are views for describing an intra prediction for a certainsub-block when the intra prediction mode of the current block is thehorizontal mode, vertical mode, and DC mode, respectively, and FIG. 13is a view for describing a process of generating a residual block for acertain sub-block on which an intra prediction has been performed.

FIGS. 10A and 10B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a horizontal mode.

FIG. 10A illustrates a sub-block before the intra prediction isperformed, and FIG. 10B illustrates a predicted sub-block generatedthrough the intra prediction on the sub-block. It is assumed that thesub-block has a block size of 4×4, is marked by a thick solid line, andincludes 16 pixels.

Referring to FIG. 10A, each pixel included in a certain sub-block beforethe intra prediction contains corresponding pixel information relatingto an original image. Each of 16 pixel information indices relating tothe original image is expressed in the form of 4×4 coordinates,“M(i,j)”, including four rows (i) and four columns (j). For example,M(2,3) indicates pixel information of a pixel located at the third row(i=2) and the fourth column (j=3).

Further, since it is assumed that the intra prediction is performed inthe horizontal mode, adjacent pixel information used for reference in anintra prediction of a certain sub-block is pixel information included ina sub-block (or macro block) adjacent to the left side of the certainsub-block and corresponds to “H(0), H(1), H(3), and H(3)”. Here, “H(0),H(1), H(3), and H(3)” refer to pixel information of four predeterminedpixels horizontally adjacent to a certain sub-block beingintra-predicted.

By intra-predicting a sub-block as shown in FIG. 10A in the horizontalmode, a predicted sub-block as shown in FIG. 10B is generated. Thegenerated predicted sub-block includes 16 pixels, each of which containspredicted pixel information. The predicted pixel information isindicated by an index expressed in the form of 4×4 coordinates,“P(i,j)”, including four rows (i) and four columns (j). For example,P(2,3) indicates predicted pixel information of a pixel located at thethird row (i=2) and the fourth column (j=3).

The predicted pixel information P(i,j) of each pixel included in apredicted sub-block, which is generated by performing an intraprediction on a certain sub-block in the horizontal mode, can beobtained by, for example, a method using the equation as defined below.In the equation defined below, H(i) indicates already encoded anddecoded adjacent pixel information.

for (i=0; i<4; i++)

for (j=0; j<4; j++)

P(i,j)=H(i);

After the predicted sub-block is generated, a residual sub-block asshown in FIG. 13 is generated by subtracting the predicted sub-blockshown in FIG. 10B from the sub-block shown in FIG. 10A. Each pixelincluded in the generated residual sub-block contains residual pixelinformation indicated by “R(i,j)”, which is obtained by subtractingP(i,j) from M(i,j).

The generated residual sub-block is then transformed, quantized, andentropy-encoded, so that the intra prediction encoding for the certainsub-block is completed. Further, by performing a dequantization, aninverse transform, and an intra-compensation on the quantized residualsub-block, a reference block is generated. Then, the generated referenceblock is used for a later intra prediction encoding of anothersub-block.

The reference pixel information of each pixel included in the referencesub-block can be obtained by a method using the equation as definedbelow. In the equation, R′(i,j) indicates pixel information included ina residual sub-block obtained by dequantizing and inverse-transformingthe quantized residual sub-block, and O′(i,j) indicates reference pixelinformation included in a reference sub-block obtained byintra-compensating and decoding such a residual sub-block.

for (i=0; i<4; i++)

for (j=0; j<4; j++)

O′(i,j)=H(i)+R′(i,j);

The above description discusses an example of an intra predictionencoding process including an intra prediction of a horizontal mode forone sub-block. Now, an example of an intra prediction encoding processincluding an intra prediction of a vertical mode for a certain sub-blockwill be discussed with reference to FIGS. 11A and 11B.

FIGS. 11A and 11B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a vertical mode.

FIG. 11A illustrates a sub-block before the intra prediction isperformed, and FIG. 11B illustrates a predicted sub-block generatedthrough the intra prediction on the sub-block. It is assumed that thesub-block has a block size of 4×4, is marked by a thick solid line, andincludes 16 pixels.

Referring to FIG. 11A, each pixel included in a certain sub-block beforethe intra prediction contains corresponding pixel information relatingto an original image. Each of 16 pixel information indices relating tothe original image is expressed in the form of 4×4 coordinates,“M(i,j)”, including four rows (i) and four columns (j). For example,M(2,3) indicates pixel information of a pixel located at the third row(i=2) and the fourth column (j=3).

Further, since it is assumed that the intra prediction is performed inthe vertical mode, adjacent pixel information used for reference in anintra prediction of a certain sub-block is pixel information included ina sub-block (or macro block) adjacent to the upper side of the certainsub-block and corresponds to “V(0), V(1), V(3), and V(3)”. Here, “V(0),V(1), V(3), and V(3)” refer to pixel information of four predeterminedpixels vertically adjacent to a certain sub-block being intra-predicted.

By intra-predicting a sub-block as shown in FIG. 11A in the verticalmode, a predicted sub-block as shown in FIG. 11B is generated. Thegenerated predicted sub-block includes 16 pixels, each of which containspredicted pixel information. The predicted pixel information isindicated by an index expressed in the form of 4×4 coordinates,“P(i,j)”, including four rows (i) and four columns (j). For example,P(2,3) indicates predicted pixel information of a pixel located at thethird row (i=2) and the fourth column (j=3).

The predicted pixel information P(i,j) of each pixel included in apredicted sub-block, which is generated by performing an intraprediction on a certain sub-block in the vertical mode, can be obtainedby, for example, a method using the equation as defined below. In theequation defined below, V(j) indicates already encoded and decodedadjacent pixel information.

for (j=0; j<4; j++)

for (i=0; i<4; i++)

P(i,j)=V(j);

After the predicted sub-block is generated, a residual sub-block asshown in FIG. 13 is generated by subtracting the predicted sub-blockshown in FIG. 11B from the sub-block shown in FIG. 11A. Each pixelincluded in the generated residual sub-block contains residual pixelinformation indicated by “R(i,j)”, which is obtained by subtractingP(i,j) from M(i,j).

The generated residual sub-block is then transformed, quantized, andentropy-encoded, so that the intra prediction encoding for the certainsub-block is completed. Further, by performing a dequantization, aninverse transform, and an intra-compensation on the quantized residualsub-block, a reference block is generated. Then, the generated referenceblock is used for a later intra prediction encoding of anothersub-block.

The reference pixel information of each pixel included in the referencesub-block can be obtained by a method using the equation as definedbelow. In the equation, R′(i,j) indicates pixel information included ina residual sub-block obtained by dequantizing and inverse-transformingthe quantized residual sub-block, and O′(i,j) indicates reference pixelinformation included in a reference sub-block obtained byintra-compensating and decoding such a residual sub-block.

for (j=0; j<4; j++)

for (i=0; i<4; i++)

O′(i,j)=V(j)+R′(i,j);

The above description discusses an example of an intra predictionencoding process including an intra prediction of a vertical mode forone sub-block. Now, an example of an intra prediction encoding processincluding an intra prediction of a DC mode for a certain sub-block willbe discussed with reference to FIGS. 12A and 12B.

FIGS. 12A and 10B are views illustrating an example of a process ofperforming an intra prediction on one sub-block in a DC mode.

FIG. 12A illustrates a sub-block before the intra prediction isperformed, and FIG. 12B illustrates a predicted sub-block generatedthrough the intra prediction on the sub-block. It is assumed that thesub-block has a block size of 4×4, is marked by a thick solid line, andincludes 16 pixels.

Referring to FIG. 12A, each pixel included in a certain sub-block beforethe intra prediction contains corresponding pixel information relatingto an original image. Each of 16 pixel information indices relating tothe original image is expressed in the form of 4×4 coordinates,“M(i,j)”, including four rows (i) and four columns (j). For example,M(2,3) indicates pixel information of a pixel located at the third row(i=2) and the fourth column (j=3).

Further, since it is assumed that the intra prediction is performed inthe DC mode, adjacent pixel information used for reference in an intraprediction of a certain sub-block is pixel information included in asub-block (or macro block) adjacent to the left side and upper side ofthe certain sub-block and corresponds to “H(0), H(1), H(3), H(3), V(0),V(1), V(3), V(3), and M”.

Here, “H(0), H(1), H(3), and H(3)” refer to pixel information of fourpredetermined pixels horizontally adjacent to a certain sub-block beingintra-predicted, “V(0), V(1), V(3), and V(3)” refer to pixel informationof four predetermined pixels vertically adjacent to a certain sub-blockbeing intra-predicted, and M refers to pixel information of a pixeladjacent in a leftward and upward diagonal direction to a certainsub-block being intra-predicted.

By intra-predicting a sub-block as shown in FIG. 12A in the DC mode, apredicted sub-block as shown in FIG. 12B is generated. The generatedpredicted sub-block includes 16 pixels, each of which contains predictedpixel information. The predicted pixel information is indicated by anindex expressed in the form of 4×4 coordinates, “P(i,j)”, including fourrows (i) and four columns (j). For example, P(2,3) indicates predictedpixel information of a pixel located at the third row (i=2) and thefourth column (j=3).

The predicted pixel information P(i,j) of each pixel included in apredicted sub-block, which is generated by performing an intraprediction on a certain sub-block in the DC mode, can be obtained by,for example, a method using the equation as defined below. In theequation defined below, H(i) indicates already encoded and decodedadjacent pixel information.

  for (j=0; j<4; j++)     for (i=0; i<4; i++)     {      k=3; //size ofsub-block =(k+1)×(k+1)      Avg=Mean [H(0), H(1), ..., H(k), V(0), V(1),..., V(k), M]; // Mean[ ] refers to averaging of values within [ ].     P(i,j) = Avg;     }

After the predicted sub-block is generated, a residual sub-block asshown in FIG. 13 is generated by subtracting the predicted sub-blockshown in FIG. 12B from the sub-block shown in FIG. 12A. Each pixelincluded in the generated residual sub-block contains residual pixelinformation indicated by “R(i,j)”, which is obtained by subtractingP(i,j) from M(i,j).

The generated residual sub-block is then transformed, quantized, andentropy-encoded, so that the intra prediction encoding for the certainsub-block is completed. Further, by performing a dequantization, aninverse transform, and an intra-compensation on the quantized residualsub-block, a reference block is generated. Then, the generated referenceblock is used for a later intra prediction encoding of anothersub-block.

The reference pixel information of each pixel included in the referencesub-block can be obtained by a method using the equation as definedbelow. In the equation, R′(i,j) indicates pixel information included ina residual sub-block obtained by dequantizing and inverse-transformingthe quantized residual sub-block, and O′(i,j) indicates reference pixelinformation included in a reference sub-block obtained byintra-compensating and decoding such a residual sub-block.

for (j=0; j<4; j++)

for (i=0; i<4; i++)

O′(i,j)=Avg+R′(i,j);

The above description discusses an example of an intra predictionencoding process including intra predictions of a horizontal mode, avertical mode, and a DC mode for one sub-block.

The image encoding according to the present disclosure as describedabove is based on H.264, and can also be performed according to a methoddefined in H.264 for another intra prediction mode other than thehorizontal mode, the vertical mode, and the DC mode.

FIG. 14 is a schematic block diagram of an image decoding apparatus 1400according to an embodiment of the present disclosure.

Referring to FIG. 14, the image decoding apparatus 1400 according to anembodiment of the present disclosure includes: a decoder 1410 fordecoding a received bit stream, so as to extract residual sub-blocks andan intra prediction mode for a plurality of sub-blocks decomposed from acurrent block; a dequantizer 1420 for dequantizing the extractedresidual sub-block; an inverse transform unit 1430 for performing aninverse transform on the dequantized residual sub-block; an intrapredictor 1440 for generating predicted sub-blocks for a plurality ofsub-blocks by performing an intra prediction according to an intraprediction mode by referring to adjacent pixel information for eachsub-block already decoded and reconstructed; an adder 1450 forreconstructing a plurality of sub-blocks by adding theinverse-transformed residual sub-blocks and corresponding generatedpredicted sub-blocks; and a block combiner 1460 for reconstructing thecurrent block by combining the plurality of reconstructed sub-blockswith each other.

The intra prediction mode for the plurality of sub-blocks describedabove is the same as the intra prediction mode of the current block.

The block combiner 1460 may combine the plurality of reconstructedsub-blocks according to either a sequence of a raster scan or a zigzagsequence.

The received bit stream described above corresponds to a bit streamencoded according to an image encoding method according to an embodimentof the present disclosure.

The image decoding apparatus 1400 according to an embodiment of thepresent disclosure may share parameters or information necessary fordecoding, dequantization, inverse-transform, and intra prediction, etc.with the image encoding apparatus 100 according to a predeterminedscheme. For example, by receiving a bit stream including information onthe intra prediction mode of the current block or information, by whichit is possible to determined the intra prediction mode of the currentblock, the image decoding apparatus 1400 may share the information onthe intra prediction mode with the image encoding apparatus 100.

FIG. 15 is a flowchart of an image decoding method according to anembodiment of the present disclosure.

Referring to FIG. 15, an image decoding method according to anembodiment of the present disclosure includes the steps of: (S1500)decoding a received bit stream, so as to extract residual sub-blocks andan intra prediction mode for a plurality of sub-blocks decomposed from acurrent block; (S1502) dequantizing the extracted residual sub-block;(S1504) performing an inverse transform on the dequantized residualsub-block; (S1506) generating predicted sub-blocks for a plurality ofsub-blocks by performing an intra prediction according to an intraprediction mode by referring to adjacent pixel information for eachsub-block already decoded and reconstructed; (S1508) reconstructing aplurality of sub-blocks by adding the inverse-transformed residualsub-blocks and corresponding generated predicted sub-blocks; and (S1510)reconstructing the current block by combining the plurality ofreconstructed sub-blocks with each other.

In step S1510, the plurality of reconstructed sub-blocks may be combinedaccording to either a sequence of a raster scan or a zigzag sequence.

The intra prediction mode for the plurality of sub-blocks describedabove is the same as the intra prediction mode of the current block.

In the image encoding method and image decoding method according to anembodiment of the present disclosure, when a current block to be encodedor decoded is predicted, the current block is decomposed into sub-blocksand each of the sub-blocks is then intra-predicted by referring toadjacent pixel information of pixels adjacent to the correspondingsub-block, instead of performing an intra prediction on each decomposedsub-block by referring to adjacent pixel information of surroundingblocks adjacent to the current block. As a result, the image encodingmethod and image decoding method according to the present disclosure canimprove the accuracy of the prediction, which can provide an image witha satisfactory reproduction quality by improved coding efficiency.

In the description above, although all of the components of theembodiments of the present disclosure may have been explained asassembled or operatively connected as a unit, the present disclosure isnot intended to limit itself to such embodiments. Rather, within theobjective scope of the present disclosure, the respective components maybe selectively and operatively combined in any numbers. Every one of thecomponents may be also implemented by itself in hardware while therespective ones can be combined in part or as a whole selectively andimplemented in a computer program having program modules for executingfunctions of the hardware equivalents. Codes or code segments toconstitute such a program may be easily deduced by a person skilled inthe art. The computer program may be stored in computer readable media,which in operation can realize the embodiments of the presentdisclosure. As the computer readable media, the candidates includemagnetic recording media, optical recording media, and carrier wavemedia.

In addition, terms like ‘include’, ‘comprise’, and ‘have’ should beinterpreted in default as inclusive or open rather than exclusive orclosed unless expressly defined to the contrary. All the terms that aretechnical, scientific or otherwise agree with the meanings as understoodby a person skilled in the art unless defined to the contrary. Commonterms as found in dictionaries should be interpreted in the context ofthe related technical writings not too ideally or impractically unlessthe present disclosure expressly defines them so.

Although exemplary embodiments of the present disclosure have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from essential characteristics of thedisclosure. Therefore, exemplary embodiments of the present disclosurehave not been described for limiting purposes. Accordingly, the scope ofthe disclosure is not to be limited by the above embodiments but by theclaims and the equivalents thereof.

INDUSTRAIL APPLICABILITY

As described above, the present disclosure can be applied to an imageencoding/decoding. When a current block to be encoded or decoded ispredicted, the image encoding and decoding method according to thepresent disclosure can improve the accuracy of the prediction, which canprovide an image with a satisfactory reproduction quality by improvedcoding efficiency.

1. An apparatus for encoding an image, comprising: a block decomposerfor decomposing a current block into a plurality of sub-blocks; and anintra prediction encoder for performing an intra prediction encoding byreferring to already encoded and decoded adjacent pixel information ofeach sub-block, based on an intra prediction mode equal to an intraprediction mode of the current block, thereby generating a bit streamfor the current block.
 2. The apparatus of claim 1, wherein the blockdivider divides the current block into the plurality of sub-blocks basedon a unit for frequency conversion.
 3. The apparatus of claim 1, whereinthe intra prediction encoder performs an intra prediction encoding onthe plurality of sub-blocks according to a sequence of a raster scan ora zigzag sequence.
 4. The apparatus of claim 1, wherein the intraprediction encoder comprises: an intra predictor for selecting a singlesub-block among the plurality of sub-blocks according to the sequence ofa raster scan or the zigzag sequence, and performing an intra predictionfor the selected single sub-block with reference to adjacent pixelinformation of each sub-block based on the intra prediction mode equalto the intra prediction mode of the current block, thereby generating apredicted sub-block; a subtractor for calculating a difference betweenthe selected single sub-block and the predicted sub-block to generate aresidual sub-block; a transform unit for transforming the generatedresidual sub-block; a quantizer for quantizing the transformed residualsub-block; and an encoder for encoding the quantized residual sub-block.5. The apparatus of claim 4, wherein the intra prediction encoderfurther comprises: a dequantizer for dequantizing the quantized residualsub-block; an inverse transform unit for performing an inverse transformon the dequantized residual sub-block; an intra-compensator forperforming an intra-compensation on the inverse-transformed residualsub-block, thereby generating a reference block; and a reference blockstorage unit for storing a generated reference block.
 6. The apparatusof claim 1, wherein the intra prediction mode of the current block isone of a horizontal mode, a vertical mode, and a Direct Current (DC)mode.
 7. A method of encoding an image, comprising: decomposing acurrent block into a plurality of sub-blocks; and generating a bitstream for the current block by performing an intra prediction encodingby referring to already encoded and decoded adjacent pixel informationof each sub-block, based on an intra prediction mode equal to an intraprediction mode of the current block.
 8. The method of claim 7, wherein,in the performing of the intra prediction encoding, the plurality ofsub-blocks are intra-prediction-encoded according to a sequence of araster scan or a zigzag sequence.
 9. An apparatus for decoding an image,comprising: a decoder for decoding a received bit stream, so as toextract residual sub-blocks and an intra prediction mode for a pluralityof sub-blocks decomposed from a current block; a dequantizer fordequantizing the extracted residual sub-block; an inverse transform unitfor performing an inverse transform on the dequantized residualsub-block; an intra predictor for performing an intra predictionaccording to the intra prediction mode with reference to adjacent pixelinformation for each sub-block already decoded and reconstructed,thereby generating predicted sub-blocks for the plurality of sub-blocks;an adder for adding the inverse-transformed residual sub-blocks and thepredicted sub-blocks to reconstruct the plurality of sub-blocks; and ablock combiner for combining a plurality of reconstructed sub-blockswith each other to reconstruct the current block, wherein the intraprediction mode for the plurality of sub-blocks is equal to an intraprediction mode of the current block.
 10. The apparatus of claim 9,wherein the block combiner combines the plurality of reconstructedsub-blocks according to either a sequence of a raster scan or a zigzagsequence.
 11. A method of decoding an image, comprising: decoding areceived bit stream, so as to extract residual sub-blocks and an intraprediction mode for a plurality of sub-blocks decomposed from a currentblock; dequantizing the extracted residual sub-block; performing aninverse transform on the dequantized residual sub-block; generatingpredicted sub-blocks for the plurality of sub-blocks by performing anintra prediction according to the intra prediction mode with referenceto adjacent pixel information for each sub-block already decoded andreconstructed; reconstructing the plurality of sub-blocks by adding theinverse-transformed residual sub-blocks and the predicted sub-blocks;and restoring the current block by combining the plurality ofreconstructed sub-blocks with each other, wherein the intra predictionmode for the plurality of sub-blocks is equal to an intra predictionmode of the current block.
 12. The method of claim 11, wherein, in therestoring of the current block, the plurality of reconstructedsub-blocks are combined according to either a sequence of a raster scanor a zigzag sequence.